A polypropylene resin is now increasingly utilized in various fields because of excellent mechanical strengths, heat resistance, machinability, cost balance, combustibility and recyclability thereof. Foamed, non-crosslinked resin moldings of a base resin including a polypropylene resin (hereinafter referred to simply as “PP moldings”), which retain the above excellent properties and which have excellent additional characteristics such as cushioning property and heat resistance, are thus utilized for various applications as packaging materials, construction materials, heat insulation materials, etc.
Recently, there is an increasing demand for PP moldings having higher rigidity and lighter weight than the conventional ones. For example, in the field of vehicles such as automobiles, PP moldings have been used in various parts such as bumper cores, door pats, pillars, tool boxes and floor mats. In view of protection of environment and saving of energy, light weight and high rigidity PP moldings retaining excellent cushioning and shock absorbing properties are desired. In the field of containers and boxes for storing and transporting foods such as fish, molded polystyrene foams have been hitherto used. Because of inferior shock and heat resistance, however, molded polystyrene foams are not suitably reused. Therefore, the need for light weight and high rigidity PP moldings is also increasing in this field.
One known method for improving rigidity of PP moldings produced by molding expanded, substantially non-crosslinked resin beads of a base resin including a polypropylene resin (hereinafter referred to as “expanded PP beads”) in a mold is to use a high rigidity polypropylene resin as a raw material. Thus, a propylene homopolymer or a propylene copolymer containing a reduced content of a comonomer such as butene or ethylene has been used. Such a high rigidity polypropylene resin, however, has a high melting point and requires a high temperature for molding. When steam is used for molding, it is necessary to use high pressure steam and, therefore, to use a special molding device having a high pressure resistance, in order to attain sufficient melt adhesion between the expanded PP beads.
Another known method for improving rigidity of PP moldings is to use expanded PP beads which show, in a DSC curve thereof, a high temperature peak of a large area in addition to an intrinsic peak which is present in a lower temperature side of the high temperature peak and is inherent to the polypropylene resin. In this case, too, it is necessary to use high pressure steam and, therefore, to use a special molding device having a high pressure resistance, in order to attain sufficient melt adhesion between the expanded PP beads.